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Fender Super Champ XD mods bypass preamp SCXD

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by xtian, Jan 6, 2012.

  1. xtian

    xtian Member

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    PREAMBLE: I wanted a lightweight 10" combo, but couldn't find a Princeton for less than $700. But I finally got one of the last SCXDs from MF, for cheap.

    This amp has a solid-state preamp with digital FX and modeling. Then the signal goes to the tube section, which is one half of a 12ax7 as a gain stage, the other half as a PI, and a pair of 6V6 as power section.

    As expected, the digital front end exhibits a lot of white noise, plus crappy digital artifacts and aliasing as notes decay into silence. Fine for loud rock, but poor for sensitive playing.

    I followed the lead of others <http://www.thegearpage.net/board/showthread.php?t=628217>, and installed a line-in jack just following the digital preamp. As I hoped, this cut all of the crappy noise from the preamp, and let me play directly into the tube stages. However, the signal from my guitar is obviously very low for this stage in an amp. It sounded nice and clean, but as if the Gain control was down around 1 or 2 (obviously, this mod bypasses all of the controls--I'm just trying to give an idea of the volume level).

    QUESTIONS: How much, in dB or mVac or something useful, does my guitar's signal need to be boosted to equate to the signal leaving the preamp of a dimed Princeton?

    I pulled out my mod, because I haven't drilled the chassis yet, so I can't test this idea: I just ordered an OCD pedal, which advertises 30dB of boost. Would that make a good preamp?

    Any other ideas for bypassing the crappy preamp and making this SCXD a poor-man's Princeton (while leaving said crappy preamp intact and available)?

    [​IMG]
     
  2. Billm

    Billm Member

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    Line level is around 1 volt. Guitar level is around 10 millivolts. So at least one 12AX7-ish gain stage is needed for guitar input.
     
  3. xtian

    xtian Member

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    Yes! Sounds good. But how does voltage equate to dB boost? Or does it?

    I've read many times that a 3dB is double the signal amplitude. Does that equate to twice the voltage? And this is logarithmic, meaning I need only 9dB boost? (9db = 10^3 boost?).
     
  4. xtian

    xtian Member

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    Well, I put the power-amp-in jack back in, this time permanently, drilling a hole in the chassis for the jack. Got out my clean boost pedal and cranked it up. EXCELLENT! Sounds great.

    Now that I know I can completely bypass the digital front end and run directly into the pure-tube section, I'm very happy about keeping this killer, little, CHEAP, multipurpose amp in the house.
     
  5. xtian

    xtian Member

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    Photos below. I put some painters tape inside the chassis to catch the metal shavings as I drilled:

    [​IMG]
    [​IMG]
    [​IMG]
     
  6. thosk

    thosk Silver Supporting Member

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  7. guitarcapo

    guitarcapo Senior Member

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    I guess as long as your clean boost pedal sounds better than the preamp Fender built in it's a valid idea. Couldn't hurt to add something like that.
     
  8. xtian

    xtian Member

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    No, you're totally right. My boost pedal is solid state, so it's not an all tube path. OTOH, it does indeed sound way better, compared to the hissy, erratic noise floor of the SCXD preamp.
     
    Last edited: Jan 7, 2012
  9. crunchman

    crunchman Member

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    Wonder if this can be done on a VCXD ? I am not great reading/interpreting schematics.
     
  10. mwnovak

    mwnovak Member

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    Friendly bump here with the same question: anyone have a lead on adding a line-in/effects-loop to the VCXD? I'm comfortable enough performing the mod, not so much with deciphering the schematic. Anyone know the correct place on the board to add the input?

    Any help is appreciated,

    --Matt
     
  11. gnobuddy

    gnobuddy Member

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    Oh, man. What I posted here before was not right. I made one huge mistake - I forgot there was a transformer inside the feedback loop! I'll correct the error in a minute, but first, for reference, here is my WRONG original post:
    I'm used to solid-state amps with no output transformer, and that led me to make a huge mistake: I forgot that the SCXD uses a transformer inside the feedback loop. While the basic concepts I used above are correct, because of my error, the numbers aren't.

    Here's the right calculation: the feedback is taken from the output of the transformer, where it drives the speaker. To put 15 watts RMS (max nominal output of the SCXD) into an 8 ohm load takes roughly 30 volts peak-to-peak, or 11 volts RMS. The two feedback resistors set the gain at about 60, so the input voltage required (at the grid of the first triode) to drive the tube amp in the SCXD to full output is roughly 0.5 volts peak to peak, or about 185 mV RMS.

    That resistive voltage divider at the input of the amp still takes its toll, meaning roughly 450 mV RMS will be needed at the output of the last opamp stage to drive the tube section of the SCXD to full output power.

    This is in fact reasonably close to line level, within a few dB.

    Because of my previous error, I was wrong about the opamps clipping before the tube amp does. The opamps can comfortably put out something around six or seven volts RMS before they clip, and the tube power amp only requires about one volt. The opamps will not clip first, the tube amp will.

    Also I'm not so sure about changing the 2.7 k resistor now. Having the transformer inside the loop changes things, while the voltage gain is around 60 from input of tube amp to speaker, it is much higher from input to the plates of the output tubes.

    For reference, I looked up Fender's official schematic for the '65 Princeton, and it uses exactly the same two feedback resistors as the SCXD - 2.7 k and 47 ohms.

    Xtian (the original poster in this thread) was definitely on the right track getting an SCXD because he wanted a Princeton - the tube section of the SCXD very strongly resembles the corresponding section of a '65 Princeton Reverb amp. Both have the same tubes, the same basic stages (triode preamp, triode concertina phase-inverter, output pentodes), and many of the resistor and capacitor values are the same as well.

    The main differences between a Princeton and the SCXD are that the Princeton has a higher supply voltage to the output tubes, and of course the Princeton has a 12AX7 doing preamp duty instead of a fistful of opamps.

    The other differences are in the effects - the Princeton uses two more tubes for reverb and tremolo, and one power-wasting tube rectifier.

    What all this means is this: if you use an external tube preamp, external reverb and/or tremolo, and possibly add one power resistor to the SCXD's power supply to simulate tube rectifier voltage "sag", the SCXD will be very, very close to a Princeton...

    Sounds like a project in the making - an electronic Cinderella story, a transformation from a grudgingly accepted budget-priced hybrid guitar amp to one of the most revered small amps in the Fender line-up!

    -Gnobuddy
     
    Last edited: Mar 28, 2012
  12. gnobuddy

    gnobuddy Member

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    Crunchman, Matt, I don't own a VCXD, but looking at the schematic, the obvious place to insert an effects loop is to remove resistor R13 and put the FX loop in its place.

    In other words, remove R13, solder shielded wire into the two holes in the circuit board left behind from removing R13, and run those two wires to the FX in/out jacks. Obviously the one coming from the opamp is FX out, the one going to the 12AX7 is FX in.

    The outer braids of those two shielded wires will need to be grounded somewhere. Probably best to ground them to a convenient point on the circuit board nearby, and use all-plastic insulated jacks for the FX loop (to avoid ground loops).

    If you do the mod this way, resistor R1 sets the input impedance of the tube part of the amp to be a rather low 4.7 kilo ohms. Any modern solid-state preamp or pedal should drive that without trouble, but it will be too low to be driven by an external tube preamp.

    You could remove R1 and replace it with a 1 mega-ohm resistor, but this will change the gain of the VCXD when you're not using the external preamp. (The signal getting to the first tube will be about five times hotter, so you'll have to back off quite a bit on the volume knob!)

    Remember that there are lethal voltages in the VCXD, and only qualified personnel using appropriate safety gear should attempt to work on the electronics...

    And let me remind everybody on this thread that electricians wear thick rubber safety gloves when working on high voltages today, and so should anyone working on a tube amp who wants to stay alive and healthy. Class 0 or Class 1 gloves will do for the voltages found in low wattage tube amps; Salisbury is one of the most respected manufacturers of the gloves. You can often get a good deal on a pair of these gloves on Ebay.

    -Gnobuddy
     
    Last edited: Mar 24, 2012
  13. mwnovak

    mwnovak Member

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    Appreciate the direction, gnobuddy, and sorry for the delayed follow-up (thought I was subscribed to this thread, but wasn't).

    Couple questions, if you have a second?

    1) Could we use a single "switched" line-in jack here, as with the SCXD mod? Or is there a reason to wire up both an in and an out at this point?

    2) What does removing that R13 (22k) resistor do to the amp if running without anything in the plugged into the effects loop?

    Good advice, and agreed: that's inexpensive piece of mind at $25-35.

    Thanks again,

    --Matt
     
  14. gnobuddy

    gnobuddy Member

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    Please don't apologise, I'm very slow to reply to email & internet posts myself.
    Will do my best!
    I was thinking FX loop, that's why I suggested separate in and out jacks. But sure, if you only want an "In" to the power amp, a switched jack could be used instead.

    It's also possible to wire up a single stereo jack as an FX loop - the way insert jacks are wired on a mixer. Tip send, ring return, or vice versa. Needs a special insert cable to work as an FX loop.

    You'd want to bypass the spot where the resistor used to be. Either use a short patch cable on the outside of the amp, from FX out to FX in, or as you suggest, use a switching jack to directly connect the output of the opamp (TP20) to the grid of the 12AX7. (I'm surprised Fender didn't put a coupling cap inline here, just to keep the opamp from possibly putting any DC voltage on the tube's grid).

    The thing is, bypassing this resistor (R13) will make the signal a lot hotter at the grid of the tube amp. That's a good thing when plugging in an external signal into the FX IN jack, but it's going to make the VCXD much louder at any given setting of the volume knob. Put another way, you'll have to dial back the volume knob to get the same level as before.
    My thoughts exactly, if tube techs had these things in 1950 they would have used them. Nobody wants to die of electrocution! :eeks

    I bought a pair of Ebay and they are rather clumsy to work with, but I'll take clumsy over dead any time!

    Good luck!

    -Gnobuddy
     
  15. gnobuddy

    gnobuddy Member

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    I know this is a late reply, but maybe this will be of use to somebody...

    Decibels are a way to measure ratios, rather than voltages. If the voltage coming out of a preamp is ten times bigger than the voltage going into it, that preamp has a voltage gain of 20 dB.

    Some other common ratios: 10 dB is roughly three:1. 40 db is 100:1.
    Errm, not quite exactly. 3 dB is in fact double the power, not double the signal amplitude. Because power is proportional to voltage squared, doubling the power only requires that the voltage increase by the square root of two - that's roughly 1.414. So 3 dB corresponds to a power doubling, or a voltage increase of roughly 42%.
    It is logarithmic, the formula is: decibels = 20 log (V2/V1) where V2 is output voltage, V1 is input voltage, and the log is to the base ten (common logarithms).

    The numbers Billm quoted are V2 = 1 volt, V1 = 10 mV or 0.01 volt. So (V2/V1) is (1/0.01), or one hundred. The log of 100 (to the base 10) is 2. Finally, you multiply this number (2) by 20 to get the answer: 40 dB!

    I'm far from an expert on tubes, but AFAIK 40 dB of voltage gain is a bit too much to expect from one conventional triode stage. It could be done with one pentode stage, or the two triode stages in one 12AX7, though.

    -Gnobuddy
     
  16. gnobuddy

    gnobuddy Member

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    I've owned an SCXD for two years now, and it's been quiet as the proverbial church mouse the entire time. No hissing or erratic noises whatsoever.

    EXCEPT for the time I plugged in one of my acoustic guitars with an onboard piezo pickup and electronics. With that one guitar plugged in, the SCXD did indeed make faint erratic hissing and grumbling sounds. Sometimes it sounded like the faint sound of bacon frying in the distance.

    Eventually one day I recognised the cadence of speech in that noise. And that was when I twigged to the cause - RF pickup! Somehow the combination of that one guitar and the SCXD was (very faintly) picking up a local radio station, and the audible effect was that faint erratic hissing/frying sound. The one day I heard the cadence of human speech, it happened to be a talk radio station.

    The weird part is this: that guitar is fine with my other amps. My SCXD is fine with my other guitars. But put that particular guitar and my SCXD together, and there's RF pickup.

    Anyhoo, all this to say one thing: if your SCXD or VCXD is making erratic hissing noises, something is wrong with it, and it's not because of the digital modeling front end - because they (SCXD's) don't all do that.

    -Gnobuddy
     
  17. mwnovak

    mwnovak Member

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    Hiya Gno. I'm getting more and more comfortable with this mod and may gather the pieces and have a go at it soon: thanks for all the feedback, I appreciate it.

    My main interest really is an FX loop, but I was thinking that the existing (factory stock) line out could be used as a send, with the line-in mod acting as a return. I thought that's how people were using this mod (as an FX loop) on the SCXD, but maybe I misunderstood? :huh

    Looking at the SCXD schematic, is that what C64 (0.1mfd cap) is doing in there? Fast getting out of my depth, but it'd be easy enough to add that cap to the VCXD while doing the line-in mod, if there's reason to add one.

    Alright, I'm starting to follow this: pulling R13 (and/or changing the value of R1) helps us better accommodate the signal coming from an external pedalboard or preamp. But if I'm only using this line-in as an FX loop--always driving the tubes from the stock preamp--I'm better off leaving R13 in place (just lifting one end of the resistor to tie in my input jack), yeah?

    Thanks again for all your feedback. I'll post pics and results if I decide to go forward with this.

    --Matt
     
  18. gnobuddy

    gnobuddy Member

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    Hi Matt, I'm currently at work and don't have the schematics in front of me. I'll look up some details once I get home. In the meantime...

    The stock SCXD line-out has some audio filtering built in (look at all those caps around the opamp driving the line-out - this is a low-pass filter). I suspect this is an attempt at speaker/cab emulation.

    So if you use the stock line-out as an FX out, and run it back into the tube amp, and then to a real speaker and cab, you've got a speaker/cab emulation running into an actual speaker/cab. Not the best recipe for good tone! You'll notice that in the stock SCXD Fender does not run the tube amp off the line-out signal, but rather taps off a signal before it goes into that speaker-emulation/line-out opamp stage.

    So this is why I don't much like that particular solution of adding just an "FX In" jack, and using the stock line-out jack.

    I don't have the VCXD schematic in front of me, but if it has the same sort of low-pass filtering before the line-out jack, then it's not really a great idea to use this as an FX out jack, IMHO.

    That's why it makes more sense to me to use the two ends of R13 as FX out/ FX in jacks. The annoying part is trying to decide whether you want to drill holes in your amps steel chassis for more quarter-inch jacks or not!

    As for C64, I don't remember the part number (schematic's at home), but I do recall the SCXD has a 0.1 uf cap between the op amp output and the grid of the first triode. Surprisingly, it seems Fender left this out of the VCXD. As you say, you could add one back in during the mod.

    Is there reason to add one? Maybe not. If the opamp behaves itself it will have near-zero output DC voltage, so direct coupling it to the triode won't hurt anything. But if you add the FX IN jack and don't include a cap, we're at the mercy of whatever guitar pedal or external preamp is plugged in to that FX IN jack. If that device is properly designed with zero output dc voltage, then we're good. If not, then that dc level will mess up the biasing on the first triode in the VCXD.

    Personally I'm uncomfortable leaving that up to chance (hoping the designer of the external pedal did her/his job properly), so I'd be tempted to put in a cap. But Fender engineers seem more sanguine about the whole thing - none of the Fender tube amps I've looked at have input coupling caps! Blues Jr., Princeton Reverb, etc - all run the grid of the first preamp triode straight to the "guitar in" jack, with no coupling cap in between. Any of those amps might be run with an external pedal or multiFX box, so Fender engineers are gambling that all these external devices will be properly designed with no DC level on their output.

    Clearly Fender has been making successful guitar amps for sixty plus years, so maybe they're right, and it's okay to take a relaxed approach to the lack of this coupling cap!
    Hmm, you got me thinking! Let me think about this and get back to you. It might be tomorrow, I have a long evening ahead of me tonight. :(

    I'm working on a little SCXD related project myself...I was given a dead one (digital board died), and I'm looking for ways to bring it back to life.

    -Gnobuddy
     
  19. djd100

    djd100 Member

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    I've used a modded SCXD for years as well, and the digital preamp has no noise issues, though it does have a very uneven sustain quotient at the very end of a sustaining note as it dies to silence. This typically isn't a problem unless you're playing solo guitar with it perhaps? I did contact Fender about fixing it, but they said "no way" LOL?

    I added the PA-In/FX Return long ago and have used the Line Out as a FX Send with no problems, but I'll look into taking the send signal from the same place Fender drives the PA instead, thanks for the heads up Gnobuddy!

    I recab'd mine into a SS Princeton 65 1 X 12 box, and typically have a MXR 10-Band EQ in the loop as the OT is small and light on bottom end. Using a 12" speaker and a touch of EQ works well for me, and this modded SCXD is a great grab and go amp that I use all the time (it gets pretty loud with an efficient 100db 12" speaker too!).

    Thanks!


     
    Last edited: Mar 31, 2012
  20. mwnovak

    mwnovak Member

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    No worries, no hurries. Only hiccup is that, the more we talk this around, the more questions I generate. :huh

    The SCXD and VCXD appear to share the same line out filtering--if the schematics going into the jack aren't actually identical, they're very close--so I see your point: it's not gonna hurt anything to use that as an FX send, but it's really not the same signal that was headed for the tubes (and that we can access at R13).

    Thankfully, I'm not squeamish about making holes.

    For the coupling cap, I honestly wouldn't know any better: I'm just thinking that, if I end up doing this mod and it's a matter of adding a cap before R13, I may as well add it while I'm in there.

    Ditto. You've got me thinking about adding a SPDT switch for R13: one position leaves it in the signal when using the input as an FX return (with the stock preamp), the other position takes it out of the signal when using the input as a preamp-in (and bypassing the stock preamp). Dunno if it's worth the trouble, but I might like the option of dumping a signal straight to the tubes.

    I may rough-out a schematic and post that up here for a sanity-check, since I think I've got a handle on this except for maybe cap and resistor values? We'll see.

    Thanks again, Gno, and good luck with your work.

    --Matt
     

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